On Mars, swiss cheese is caused by sublimation of nearly pure CO2 (there's probably some dust contamination) sublimating into the atmosphere due to solar heating. On Mercury, I'd guess the sublimating volatile is less pure, and the halos around the hollows are due to heavier materials being redeposited. I'd also guess that the sublimation is driven not solely by direct insolation - i.e., the whole surface gets pretty hot. That might account for the differences in morphology.

Yes, fantastic examples. If you didn't say Moon, I would have said 'Mars'. Is it possible, for some sort of, for want of better words, 'soil liquefaction',without water being the liquid medium. Caused either by impact, or interiorseismic activity. It's coming up to Australia Day, so here's a from me.

One idea used for the lunar examples, dating back, I think, to Peter Schulz in his classic 'Moon Morphology' book, is regolith excavation and 'liquefaction' to some extent, by the occasional release of residual volcanic gases or slowly accumulating radiogenic argon. Gas-supported regolith (liquified, if you like to call it that) might account for the sharp meniscus-like boundary on most of them, as the regolith flowed back into the hollow at the end of the eruption. This idea is mentioned in my LPSC abstract. This can work just as well on Mercury - the MESSENGER team is saying there is more volatile content than expected in Mercury. (Volatiles might include sulphur compounds rather than water).

My point here - the MESSENGER folks have said several times now that these hollows are not found on the Moon. They certainly are much more numerous on Mercury, but I believe similar features are seen on the Moon, and any explanation for Mercury ought to be at least considered for the Moon as well. And on the Moon we can explore them directly - even a GLXP team could look at one.

Here's what it looks like to me. There is a layer just below the surface that easily collapses (evaporates?) at the least provocation. When could such a layer have been laid down? When Mercury had an atmosphere. Could the same apply to the Moon? Why not?

If Earth were suddenly denuded of its atmosphere would hollows form? If so, where and why?

The hollows look to me like they formed from the partial sublimation of a smooth mantling material at the surface. If they are formed by sublimation, then a clue to what the mantling material consists of probably lies in Mercury's exoshere: sodium, potassium, calcium..., hum, powdered igneous rock? Particularly felsic igneous rock?

One idea used for the lunar examples, dating back, I think, to Peter Schulz in his classic 'Moon Morphology' book, is regolith excavation and 'liquefaction' to some extent, by the occasional release of residual volcanic gases or slowly accumulating radiogenic argon. Gas-supported regolith (liquified, if you like to call it that) might account for the sharp meniscus-like boundary on most of them, as the regolith flowed back into the hollow at the end of the eruption. This idea is mentioned in my LPSC abstract. This can work just as well on Mercury - the MESSENGER team is saying there is more volatile content than expected in Mercury. (Volatiles might include sulphur compounds rather than water).

My point here - the MESSENGER folks have said several times now that these hollows are not found on the Moon. They certainly are much more numerous on Mercury, but I believe similar features are seen on the Moon, and any explanation for Mercury ought to be at least considered for the Moon as well. And on the Moon we can explore them directly - even a GLXP team could look at one.

Some of those raised blobs in the crater look like the result of surface tension - something that you might see after a spill of molten metal had solidified.

QUOTE (schaffman)

The hollows look to me like they formed from the partial sublimation of a smooth mantling material at the surface.

I was thinking along the lines of glass. Maybe these are areas where a thin surface layer supercooled, and as they thermally cycle, there are areas of crystallization? A different sort of phase change than sublimation.

I think gasses are far more likely to be involved, but hey, crazy ideas are fun.

The hollows look to me like they formed from the partial sublimation of a smooth mantling material at the surface.

Exactly. Mantling as in descending from above rather than rising from below. We see hollows on top of hills as well as in low spots. The place this reminds me of is Io. Volatiles such as sulphur, oxides of sulphur and others are continuously mantling that world. If Mercury experienced an Io-like episode followed by a settling of rocky dust perhaps that could provide the circumstances for the hollows to form, under the very non-Io-like conditions that prevail on Mercury.

Looking back at the Quickmap interface mentioned above... you can now turn on a layer of high resolution mosaics. They appear as white boxes when zoomed out, and the detail appears as you zoom past a certain level.

I was thinking along the lines of glass. Maybe these are areas where a thin surface layer supercooled, and as they thermally cycle, there are areas of crystallization? A different sort of phase change than sublimation.

I think gasses are far more likely to be involved, but hey, crazy ideas are fun.

Wow. I hadn't thought of glass. That's interesting. Perhaps some type of devolatilization of impact melt sheets? I was originally thinking of fine-grained crater ejecta, but since glass is largely amorphous, it probably melts or gives off volatiles more easily than crystaline silicates. Just a guess.

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